Modulation system and method



A* mi maa/75mm March 25, 1952 Filed Nov; :ao/l 1947 W. E. BRADLEY MODULATION SYSTEM AND METHOD 5 Sheets-Sheet l INVENTOR.

BY CLM M15@ March 25, 1952 Filed Nov. 20. 1947 w. E. BRADLEY 2,590,373

MODULATION SYSTEM AND METHOD 5 SheQtS-Sheet 2 ro ma L] INI/NTOR. wIlL/H07 C. Eff/9WD March 25, 1952 Filed Nov. 20. 1947 SMS2 NTQQFG Patented Mar. 25, 1952 MODULATIN SYSTEM AND METHOD William E. Bradley, Springlield Township, Montgomery County, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application November 20, 1947, Serial No. 787,043

16 Claims.

The invention herein described and claimed relates .to improved methods of and means for producing modulated carrier wave signals. More specifically, it is intended to provide a method of and means for effecting highly linear modulation of high frequency carrier wave signals, particularly in the so-called microwave range, by

lated signal having a carrier frequency in the y microwave range. This is particularly desired because of the more ready availability of channels in that range, the high directionality of antennas which can be constructed for operation at such frequencies and the favorable propagation characteristicsat these frequencies. All of these factors render operation in this range desirable for numerous applications and particularly in the relaying of wideband intelligence between more or less distantly spaced transmitting and receiving stations. This latter application has arisen particularly in connection with the need for means to relay televisionsignals between cities displaced, one from the other, by relatively large distances.

The means presently available for generating power at frequencies in the microwave range are relatively limited in number and, to date, have proven somewhat iniiexible in their application. For example, a principal source of high power oscillations in the microwave range at the present time is the Well-known cavity magnetron. Although this device possesses the advantage of being able to generate large amounts of power at frequencies in the microwave range, the means which have been developed in the past to effect modulation of the output energy from a magnetron oscillator have been subject to a number of serious shortcomings.

It is known .to pulse-modulate the output from a magnetron oscillator by the simple expedient of varying the anode voltage thereof in response to a signal comprising time-spaced electrical pulse signals. Moreover, it is known to vary the spacing of the modulating pulses in response to intelligence desired to be transmitted, whereby to produce a high power, microwave frequency, carrier wave signal, modulated in response to such intelligence. This method of modulation, however, possesses the disadvantage that it is extremely limited as to the frequency bandwidth of the intelligence which can thus be transmitted. For example, it has been impossible, in this manner, to modulate a magnetron oscillator with intelligence comprising a band of (lil frequencies as broad as that required for television purposes (i. e. 4-megacyles).

More recently, also, magnetron oscillators have been developed whose output frequencies can be deviated throughout comparatively wide ranges. For example, it is known to construct magnetrons capable of oscillating at frequencies of the order of 4,000 megacyles, deviable throughout a range as large as 8 megacycles and presenting a high input impedance to the signal applied to effect deviation. These results are achieved using arrangements in which electron beams of controllable intensity, in addition to the one which participates in the production of oscillations, are caused to flow in one or more of the magnetron cavities in paths which are essentially spiral about axes perpendicular to the direction of the electric fields within said cavities and in regions where said lields are of greatest intensity. Varying the intensities of these beams produces variations in the tuning of the cavities, and thus varies the frequency at which the magnetron oscillates. Unfortunately, the linearity of the relationship between the applied modulating signal and the frequency of the output signal generated by such magnetrons is not adequate for many applications.

Accordingly the principal objects of the invention are the following:

.(1) To provide a method of and means for effecting frequency modulation of carrier wave signals in the super-high frequency or microwave range by relatively wideband modulating signals, with a high degree of linearity and so as to produce, with high efliciency, a large output of modulated power; l

(2) To provide an improved method of and means for frequency-modulating oscillatorsparticularly cavity magnetrons-with a high degree of linearity;

(3) To provide an improved method of and means for converting a frequency-modulated carrier wave signal of relatively low carrier frequency into a similarly modulated carrier wave signal of appeciably higher carrierfrequency; and

(4) To provide a method of and means for converting a frequency-modulated carrier wave signal of appreciably higher carrier frequency; larly modulated carrier wave signal of a different carrier frequency, without the necessity for first demodulating the original carrier wave signal,

said method and means being especially suited for use in high frequency radio relay chains employing a plurality of relay stations, each receiving a modulated carrier Wave signal of a particular carrier frequency and transmitting a similarly modulated carrier wave signal of a different carrier frequency.

As is well recognized, if two electrical wave signals of specified frequencies are mixed or heterodyned by the employment of any known form of there results a signal comprising a spemrum of components which, among others', include components of various frequencies which are, respectively, the sums and differences of the higher of the original frequencies and an integral multiple of the lower of said frequencies. If one of the original signals (e. g. the lower in frequency) is frequency-modulated, a similar spec'- trum results in which the component, whose frequency is that of the original signal of higher frequency, is of substantially constant frequency, while the components corresponding to the sums and differences of the higher original frequency and integral multiples of the lower (modulated) frequency vary in frequency in substantial accordance with the variations in the latter frequency. This phenomenon provides the basis for a method of converting a frequency-modulated carrier wave signal of a particular carrier frequency into a similarly modulated carrier wave signal of a different carrier frequency. Each of the sideband components (whose mean frequency is equal to the sum or difference of the higher original frequency and an integral multiple of the means value of the lower, modulated original frequency) will be frequency-modulated in some degree in accordance with the modulation of the original signal of lower frequency. ln general, the variations in frequ'ency of the sideband components will be proportional to those of the original frequency-modulated signal, but the amount of deviation in frequency will differ depending on the order of the sideband. Thus, the deviations of the nrst upper and lower sidebands may be about the same as the deviations of the original signal, while the deviations of the higherorder sidebands may be substantially greater. The energy comprised in any one of these components may be utilized as the desired frequencymodulated output, whose mean frequency diners from that of the original modulated signal.

in thus utilizing the phenomenon, however, it is to be borne in mind that the energy represented by any one of these components will be small compared to the total energy represented by all of the components in the spectrum. Moreover, in general -it will be substantially less than the component whose frequency is relatively fixed and equal to the higher of the original frequencies. Accordingly, if, as proposed above, one of the sideband components is used as output, the efficiency of the system will be comparatively low.

It is a feature of the invention that it makes it possible to overcome this disadvantage of heterodyne modulation and to provide modulating systems which incorporate the advantages of this method and which at the same time are highly efficient.

According to the invention it is proposed, by suitable means, to lock, or to maintain substantially fixed, the frequency of one of the sideband components, which, as above pointed out, will normally vary in frequency in the same manner as the lower in frequency of the input signals supplied to the mixer. When this is done, the carrier component of the spectrum will no longer continue to remain fixed in frequency, but will itself vary in some degree in accordance with the modulation of the lower in frequency of the input signals to the mixer, the degree of variation being dependent upon several factors, including the effectiveness of locking of the particular sideband component. This being the case, the carrier component may now be used as the desired frequency-modulated output, and, since the fraction of the total power in the spectrum represented by this component is substantially greater than that represented by any of the sideband components, the emciency of the system will be correspondingly enhanced.

As contemplated by the invention, the mixer may comprise an oscillator adapted to oscillate at a frequency corresponding to the higher in frequency of the signals to be mixed. This oscillator may be of a form which is adapted to be amplitude-modulated in response to a frequencymodulated signal corresponding to the lower in frequency of the input signals to be mixed. lf there exist certain conditions, presently to be specified', there will appear, in the output or tank circuit of the oscillator, the spectrum hereinbefore referred to, in which the fundamental or carrier component is of the frequency of the higher of the input signals and is substantially fixed in frequency,` while the sideband components have mean frequencies equal to the sums and differences of the higher input frequency and integral multiples of the lower input frequency, and vary in frequency in accordance with the modulation of the lower input frequency. The principal condition precedent to the production of this desired spectrum is that 4the oscillations generated by the oscillator, when it is modulated-, must be phase-cohered-that is, the phase ofthe' oscillationsV at any instant must be governed bythe phase of oscillations that existed at any previous instant. This condition is readily obtained if the amplitude of oscillations is never permitted to decay below a certain level which is dependent upon various factors including, for example, the noise level in the system and the magnitudes of spurious signals existing therein.

When such an arrangement is employed, the means used to lock a particular sideband component and maintain its frequency substantially invariant may, for example, comprise an auxiliary tuned circuit or tank, of relatively high Q compared to that of the oscillator, tuned to the mean frequency of the sideband to be locked, and coupled to the oscillator tank circuit. The effect of this auxiliary tank is to overcome, substantially, variations in the frequency of the particular sideband component, thereby causing the fundamental or carrier component of the spectrum to assume these variations.

Alternatively, locking may be effected by means of a separate oscillator, oscillating at the mean frequency of the sideband component to be locked and coupled to the oscillator tanl; circuit so as to supply energy thereto.

That the method is particularly well adapte: for effecting modulation of the output energy from a cavity magnetron oscillator will, in view of the foregoing, be readily apparent. Thus, to the cathode of the magnetron there may be supplied a frequency-modulated carrier wave signal Whose mean frequency is lower` than the frequency of oscillation of the magnetron. The application of this signal in this manner will produce effective variations in the conductance of the magnetron oscillator circuit. The amplitude of the signal should be such, and it should be applied in such manner, that the oscillations produced in the magnetron output cavity will be phase-cohered, in the sense previously defined. It has been determined that this condition can be satisfied even when themagnetron is operated class C at a relatively low duty cycle. The output will then contain a component of relatively large magnitude, substantially invariant in frequency and having a frequency equal to a nori mal frequency of oscillation of the magnetron, together with sideband components whose mean frequencies are equal to the sums and differences of the magnetron oscillation frequency and in tegral multiples of the mean frequency of the modulating signal, and which vary in accordance with the variations in frequency of the latter signal. However, there are also provided appropriate means, in the form of the auxiliary tank circuit or oscillator hereinbefore mentioned, coupled to the magnetron cavity so as to maintain substantially xed the frequency of one of the sideband components, whereby the frequency of the fundamental or carrier component isw caused to vary in accordance with the modulating signal (i. e. according to the frequency variation of the signal applied to the cathode of the magnetron to drive it).

Not only is the method adapted for use with magnetrons of the more conventional type in the manner above set forth, but it is usable to advantage with the more recently developed electron deviator types, hereinbefore referred to, in which the frequency of oscillation is deviable throughout comparatively wide ranges through the agency of electron beams of controllable in tensity flowing in the cavities of the magnetron. In utilizing a magnetron of the electron deviator ,type in accordance with the invention, the input, frequency-modulated, carrier wave signal, instead of being applied to anode-modulate the magnetron (and thereby produce modulation ol" the conductance of the oscillator circuit) as in the case above described, is applied to the grid or grids controlling the intensities of the electron beams in the magnetron cavities. Thus the capacitance of the equivalent oscillator circuit is modulated, the frequency of oscillation of the magnetron is dcviated in response to the frequency-modulated input signal, and there is produced an output spectrum comprising at least some components whose frequencies vary in response to the variations in frequency of the input signal. Locking of one of these components is effected by either of the aforementioned exl pedients of using an auxiliary tank tuned to the mean sideband frequency, or using a separate c'w oscillator supplying energy to a cavity of the magnetron at the mean frequency of the sideband desired to be locked.

This last described method of applying the invention illustrates feature thereof which is of considerable significance-namely, that the oscillator to be heterodyne-modulated need not necessarily be amplitude-modulated by the lower frequency intelligence-*modulated input signal. On the contrary, th-e oscillator may be modulated, in response to this signal, in amplitude, frequency or phase, or in any convenient combination of these three. Any one, or combination, of these methods of modulation will result in the production of an output spectrum comprising a plurality of sideband components, at least some of which will vary in frequency in response to variations in the frequency of the input signal. Accordingly the frequency of one of these normally variable components may be locked by one of the methods aforementioned, and the frequency of another of the components will thereby be caused to vary in resp-onse to the frequency modulation of the input signal, and can be employed as useful, frequency-modulated output.

Thus it will be apparent that, although the method is by no means confined, in its appli cation, to the magnetron type of oscillator, it is, nevertheless, a very important feature of the invention that it provides a meth-od of utilizing a magnetron for purposes forwhich, heretofore, it has not been considered to be suited-namely, the achievement of highly linear and eicient modulationof large quantities of power in the super-high frequency or microwave range.

Accordingly, in its broader aspect, the invention may be regarded as comprising a modulating system consisting of: a source of a signal of predetermined frequency, a source of a second signal of modulated frequency, means for mixing said first and second signalsvto yield a third signal, said third signal comprising a plurality of frequency components, at least one of which tends to be frequency-modulated in some degree in accordance with the frequency-modulation of said second signal, means for substantially reducing the tendency toward variation in the frequency of said one component, whereby there is imparted to another of said components frequency variations according in some degree to the modulation of said second signal, and means for deriving an output signal which varies in fre quency in accordance with the frequency varia tions of said other component.

The method of the invention, the manner in which it may be practiced, and the details of construction and operation of representative embodiments in accordance therewith, will be more fully appreciated from a consideration of the following specication together with the accompanying drawings, in which:

Figure 1 is a basic equivalent circuit of a heterodyne modulated oscillator according to the in vention,

Figure 2 is the equivalent circuit of a typical practical embodiment of the invention as illustrated in Figure 3,

Figure 3 illustrates, partially schematically and partially diagrammatically, one embodiment of a heterodyne modulated oscillator according to the invention in which locking of a particular sideband component in the output from a magnetron oscillator is effected by means of an auxiliary cavity resonator coupled to the magnetro output, l

Figure 4 is a fragmentary sectional perspective view taken along the line 4-4 of Figure 3,

Figure 5 is the equivalent circuit of a constant impedance filter comprising a portion of the embodiment of Figure 3.

Figure 6 illustrates a-n arrangement for compensating for incomplete locking of the sideband component in the output of 'a heterodyne frequency-inodulated oscillator, which may result when the magnitude of the sideband component is made relatively small compared to the fundamental component in order to secure unusually high efliciency. n

Figure 7 illustrates an embodiment of a heterodyne modulated oscillator according to the invention, in which locking of a particular sideband component in the output of a magnetron oscillator is effected by injection of a continuous Wave signal from a separate source into a cavity of the magnetron, and j Figure 8 illustrates an embodiment of the ini vention incorporating an intermediate frequency amplifier for supplying the input modulated carrier wave signal to the oscillator which is to be heterodyne modulated in response thereto, and adapted to predistort the signal thus supplied in a manner to compensate for a tendency which may exist for the output signal from the oscillator to include variations which are a function of the rate of change with time of the frequency of the input signal thereto.

Referring now to Figure l,v which illustrates the equivalent circuit of an elementary form of a heterodyne-frequency-modulator circuit employing a magnetron oscillator in accordance with the invention, the magnetron oscillator itself is represented by the tuned circuit I, having losses, represented by shunt conductance Gio, and shunted by a variable conductance 3 to simulate the variable conductance of the magnetron oscillator, which must be capable of assuming at least negative values. The tank circuit I cf the oscillator is coupled, through a suitable coupling admittance 4, to an auxiliary parallel resonant circuit tuned to the frequency of the sideband, in the output of the conductanceY modulated oscillator, which it is desired to hold fixed. Modulation of the oscillator is effected by varying conductance 3 in response to a frequencymodulated input signal. The carrier and sideband components hereinbefore referred to, as determined by the characteristics of the input signal and the frequency of oscillation of the magnetron, appear across the magnetron tank cir-- cuit I.

Practically, however, the circuit of a modulator in accordance with the invention may not be as simple as that represented by the equivalent circuit of Figure l. Rather the equivalent circuit of a practical heterodyne frequency modulator may be as represented in Figure 2. Here, as in Figure l, the magnetron is represented byl a parallel tuned circuit II shunted by a variable conductance I2, the magnitude of which varies in accordance with the instantaneous value of the modulated input signal. The oscillator tank circuit is coupled, by means of a transmission line section I3, to a signal utilization circuit I4 having an impedance Zr.. As illustrated, the left-hand end of this. transmission line section may be inductively coupled to the oscillator tank circuit. At a point interjacent the ends of transmission line section I3, there may be inductively coupled thereto, the auxiliary tank circuit I5. If, as illustrated in the figure, the effective electrical length of the portion of transmission line section I3, between the coupling to oscillator tank II and the point of coupling between auxiliary tank I5 and the transmission line section, is an odd number of half-wavelengths, at aj frequency approximating that of the oscillations, then this portion of the transmission line section may be omitted completely, and the equivalent circuit of Figure 2 is reduced toa circuit which is very similar in form to that of Figure 1. Attention is here invited to this equivalence inasmuch as it will be of assistance later in explaining the operation of an embodiment ofthe invention.

The embodiment of the invention illustrated in Figure 3 may be represented by an equivalent circuit corresponding substantially to that of Figure 2. In Figure 3, the output of a cavity magnetron oscillator 2l is supplied through a length of coaxial transmission line 22, of suitable characteristics, to one end of a waveguide section 23. The coupling between transmission line section 22 and waveguide 23 may be capacitive and is effected in conventional manner by permitting the internal conductor 22a of line section 22 to extend approximately halfway into waveguide 23 in a direction normal to the larger cross-sectional dimension of the guide, as illustrated. At its other end, the waveguide section is provided with a Y-junction 24 which feeds separate waveguide sections 25 and 2l having cross-sectional dimensions which are preferably almost equal to those ofv section 23. One of these sections, 25, is terminated in an antenna or other useful load 26, while the other section, 21, is terminated in a dummy load 28.

Coupled to waveguide section 23, at a point interjacent its juncture with transmission line section 22 and its termination in Y-junction 24, is an auxiliary cavity resonator 29, tunable by means of tuning screw 29a. The degree 0f coupling is controllable through the agency of an adjustable iris 39 formed by slidable members 3| and 32, adjustable through the cooperation of screws 33 and 3ft, as shown in detail in the fragmentary sectional perspective View of Figure 4. In the embodiment illustrated, the coupling is magnetic by reason of the arrangement of the resonator with reference to the waveguide so that the magnetic lines of force in both resonator and waveguide, in the vicinity of the iris, are essentially parallel (TELO mode of propagation in the waveguide). However it is to be understood that similar results would be obtained using capacitive coupling. The length of that portion of waveguide section 23, between itsv coupling to resonator 29 and its juncture with transmission line section 22, is adjustable by means of a linestretcher 35. This is formed conventionally by providing elongated slits 35a in opposite walls of the waveguide to permit small variations in the spacing of the other opposing walls of the guide to be effected by turning screw 35d in clamp member 35o. Similarly, that portion of waveguide 23 between the coupling to resonator 29 and its termination in `injunction 2li, is adjustable in length by means of a similar line-stretcher 36. Immediately adjacent the 1l-junction there is also provided a tuning screw Iii for the purpose of modifying the voltage standing wave ratio in the waveguide section, for reasons which will be explained hereinafter.

WaVeguide sections 25 and 2i are provided, respectively, with branch portions 3'? and 38, which are separately variable in effective length through the agencies of slidable pistons 39 and 40. Each of these branch sections is preferably displaced electrically, from the effective center of Y-junction 24 an integral number of half wavelengths at the fundamental frequency of magnetron oscillator 2 Ij. The effective electrical length of section 31 is adjusted to be an integral number of half wavelengths at the magnetron frequency; while that of section 38 isl ina-de substantially equal to an odd number of quarter wavelengths at the same frequency. As a result of these adjustments waveguide sections 25 and 2, and their associated sections 3'! and 3S, will cooperate with dummy load 2S, when the impedance of the latter is made equal to that of useful load 25, to provide, in eifect, a bandpass filter interposed between Y- junction 2li and antenna or useful load 2t. The filter thus formed will present a substantially constant input impe-dance at Y-junction 2d and may be adjusted so as to transmit to the antenna or useful load the fundamental frequency component in the output from magnetron oscillator 2|, to which will have been imparted variations substantially corresponding to those of the frequency-modulated I.F. input signal, as will be explained hereinafter. It should also be adjusted to transmit neither the locked nor the other sideband components.

The equivalent circuit of the filter is illustrated in Figure 5. In' this circuit,

of Figure 3, while parallel tuned circuit 5S repre- 4 sents waveguide section im of Figure 3 and has an impedance Z1. The product of Z1 and 4Z2 is made equal to Z02, whereupon the impedance looking into the network is equal to Zo, the load imped--l ance.

Magnetron 2|, which may be a conventional c-w magnetron operating in the S, X or K band, is driven by a suitable driver tube, which, in this instance, is pentode d2. The plate of tube i2 is coupled through condenser it to the cathode of the magnetron, and the cathode leads supplying heater power thereto may include a binlar winding "it which acts as an R.F. choke at the frequencies comprising the signal supplied to the magnetron to drive it. Also, there may be included a resistor de, connected between the magnetron cathode and ground, which cooperates with the inherent capacitance of the cathode to provide the necessary bandwidth in the coupling circuit between the driver tube i2 and magnetron 2|.

The driving signal, which may be a frequencymodulated intermediate frequency carrier derived from the output of an intermediate frequency amplifier (not shown) is supplied through coupling condenser el to the control grid. of pentode di?. From theoretical considerations it might be thought that most efficient operation of the magnetron would be obtained if the magnitude of the driving signal were adjusted so as to produce class C operation of the magnetron (i. e. so that oscillations build up in the magnetron during only a relatively small portion of each I.F. cycle). Actually this may not be the case because of the relatively large amount of power which is normally required to drive the magnetron class C. Accordingly, for the achievement of optimum over-all efficiency, it may be preferable to adjust the magnitude of the I.-F. input signal applied to the control grid of tube ft2 so as to cause less violent modulation of the magnetron (i. e. so that the magnetron osoillates during a relatively large portion of each I.-F. cycle).

The frequency-modulated input signal applied to drive the magnetron will cause oscillations to build up in the magnetron intermittently at a rate which varies in accordance with the frequency modulation of the driving signal. As a result of this mode of operation, there will exist, in the magnetron output cavity, signal frequency components corresponding to the normal frequency of oscillation of the magnetron, as well as sideband modulation components corresponding to the sums and differences of the normal frequency of oscillation of the magnetron and integral multiples of the mean or carrier frequency of the frequency-modulated driving signal. As already mentioned heretofore, the norkmal tendency will be for the fundamental or carrier frequency (corresponding to the natural frequency of oscillation of the magnetron) to remain substantially constant, while the frecordance with the frequency modulation of the driving signal. However, in the arrangement illustrated in Figure 3, owing to the presence of auxiliary cavity resonator 29 coupled to the magnetron output cavity through the agency of waveguide 23 and transmission line section 22 and tuned to the mean frequency of one of the sideband components, a substantial force will be exerted tending to overcome the variation in frequency of that particular sideband component and tending at the same time to produce similar variations in the frequency of the fundamental or carrier component of the magnetron output. Ithas been determined that such locking of a sideband component will take place where the effective electrical length of the coupling circuit between the output cavity of magnetron 2| and cavity 29, comprising transmission line section 22 and the left-hand portion of waveguide section 23, is equal to an integral number of half wavelengths. Actually it appears that this is not a critical requirement and that this length may be varied considerably without adversely affecting the ability of cavity resonator 29 to lock the selected sideband component in the output from magnetron 2|.

The effectiveness of the auxiliary cavity resonator in locking the selected sideband in the output spectrum of a heterodyne frequency-modulated oscillator is of primary importance in determining the efficiency of operation according to the method of the invention. This effectiveness is controlled by several factors in addition to the one just mentioned. To appreciate more fully the nature of these factors and their effect, it will be helpful to refer again to the equivalent circuit of Figure 1.

Before proceeding with a discussion of the several factors which iniiuence the effectiveness of locking, it will be desirable to denne certain terms presently to be used in discussing the equivalent circuit of Figure 1 as follows:

Tuned circuit will be referred to as the primary.

Tuned circuit 5 will be referred to as the secondary.

Loaded primary refers to the condition of tuned circuit when points E and F are shortcircuited.

Loaded secondary refers to the condition of tuned circuit 5 when points D and F are shortcircuited.

Unloaded primary refers to that portion of tuned circuit comprising L10, C10 and the portion of G10 which represents unavoidable losses of the circuit (i. e. G10 is to be regarded as comprising the inherent conductance of the tuned circuit plus any conductance which may be introduced incident to extracting power or modifying bandwidth).

Unleaded secondary refers to the condition of tuned circuit 5 when the resistive component of coupling admittance Yiz is infinite.

w11=resonant frequency of loaded primary, o22=resonant frequency of loaded secondary, y11=half bandwidth of loaded primary, ^/22=half bandwidth of loaded secondary, wmzmodulating frequency,

Q10=Q of unloaded primary,

QiizQ of loaded primary,

Q20=Q of unloaded secondary, and

Q2z=Q of loaded secondary.

Q, in accordance with conventional notation, designates the ratio `of reactance to resistance of a particular circuit.

rEhere follows an enumeration and discussion oi reach of the factors principally affecting the effectiveness of locking. Y

1) o of the loaded primary roi' best locking, the a f 'the loaded primary' should be as low asis consistent with the requirement to sustain oscillations.

(2) Q of the loaded secondary This should b e as high jas possible, but is limited bythe fact that the loaded Qcannot exceed the unloaded Q, which, in turn, is. inherently limited. It is also to be noted that the {Qof the loaded secondary will be reduced depending upon the magnitude of theconductivecomponent of the coupling admittance. As will. presenty become apparentit is desirable that -there be a conducr11-.n2 Although this verpression is derived"from a ccnsideration of an oscillator modulated class C at a relatively low duty cycle, experiment shows that good results are obtained by following it in casesY where the modulation is less violent 'and where the dutycycleis relativelyhi'gh. The phase angle o maybe Acontrolled by appropriate selection of the magnitude of theconductve and susceptive components of the coupling admittance. How ever, as mentioned (2) above, the conductiv component cannot be increased beyond a certain value without .undulyloading the secondary. n the circuit according to Figure 3, the magnitude and phaseY of the .effective coupling admittance between the output cavity of magnetron 2l and auxiliary cavity 29 is adjustable by means of line stretcher 36 andtuning screw 4l. The former, ashaswalready b'eenmentioned, is effective to vary the electrical-length of that portion of waveguidesection 23 between cavity resonator 29 Ye-iunction 24, whilel the latter is adapted to controlthejconditions of impedance mismatch in the vicinity -of the Y-junction.

n 4) Impedance llevel of the secondary 1f the L/C ratio of the secondary were zero, the secondary, even at resonance, would present a short circuit between points E and F in Figure l and hence would exert no influence on the oscillator. If, on the otherhand, the L/C ratio were very large, the secondary would be unduly loaded and would be incapable of effectively locking a sideb'and. 1t has been determined that, when the phase angle of the coupling admittance is that specified by Equation l, the impedance level is optimum for sideband locking when the following condition is satisfied:

Vis derived from consideration of an oscillator modulated class C at a relatively low duty cycle, but it has been ascertained that it serves as a 12 satisfactory guide in other cases as well. YAdjustment of the impedance level to satisfy this condition is made in the arrangement according to Figure 3 by variation of the magnetic coupling between waveguide section 23 and cavity resonator 29 through variable iris coupling (5) Frequency and bandwidth of modi/.latino signal lThe mean frequency of the frequency-modulated input signal which drives the oscillator determines the spacing of the frequency compoand secondary. In general, the lower is this mean frequency, the better will be the locking of a par ticular sideband. If, however, a wideband ir.- telligence is to be transmitted, the modulating frequency must be relatively high to permit satisfactory design of amplifiers through which the modulating signal must pass before being used to drive the oscillator. Moreover the modulating frequency cannot be limited'to too low value without unduly restricting the usefulness of the system as a frequency converter. In typical cmbodiments, satisfactory locking will obtain if the modulating frequency is made of the order of the half-bandwidth of the loaded primary.

At the same time, the bandwidth of the moduiating signal cannot be increased unduly without increasing distortion of the output signal. It is preferable, for example, that it not exceed onetenth the half-bandwidth of the loaded prmlary. In the case of a magnetron, the half-bandwidth increases as the frequency of operation of the magnetron increases. Hence, if the inputv signal comprises a wide band of frequency components, it is.preferable to use a magnetron adapted for operation at a higher frequency than if only a narrow band input signal is involved. Thus, for example, if television signals are involved, satisfactory results will more readily be obtained using an X-band magnetron than by using an S- band magnetron. However, it should be understood that even an S-band magnetron could, un der certain circumstances, be made toV operate satisfactorily in this application if pains are taken to insure suitably high Q of the secondary circuit.

It will be apparent that, in order for a heterodyne frequency-modulated oscillator to operate at high efficiency, it will be desirable to have the carrier frequency component of the output signal .spectrum as large as possible, while minimizing the magnitudes of the sideband components, including the sideband to be locked. However, it appears, as corrollary to the above criteria for optimum locking of a selected sideband, that satisfactory locking cannot ordinarily be achieved ii" the power represented by the sdeband to be locked is made unduly small compared to the power represented by the carrier component. If the magnitude of the selected sideband component is reduced beyond a certain point it will be impossible, by either of the aforementioned methods, to hold it essentially fixed in frequency in order that its variations may be imparted to the carrier component. The greater the reduction eyond this point of the power represented by the sideband component, the more will the frequency of the component tend to vary in accordance with variations in frequencyof the modulated intermediate frequency input signal, and the less Awill these variations tend to be imparted to the carrier component of the output signal. This result may be regarded as a form of distortion in the modulated carrier wave signal produced. If the variations in frequency of the locked" sideband component are linear with respectI to the variations in frequency of the modulated input signal, this distortion will not be objectionable, but vill amount merely to a gain or loss in deviationof the frequency-modulated output signal compared to that of the frequency-modulated input signal. A loss in deviation might prove to be of some disadvantage in the case of a high frequency relay chain comprising a plurality of relay stations each employing heterodyne frequency-modulation. After transmission through a number of such stations, the intelligence contained in the original frequency-modulated signal would, to a large extent, be dissipated unless some special steps were taken to preserve it.

It has been discovered that, in a heterodyne frequency modulation system, the efficiency can be raised appreciably by delegating a considerably greater amount of power to the carrier frequency component of the output signal, While correspondingly reducing the amount of power in the sideband component to be locked, and at the same time avoiding the disadvantages of nonlinear distortion and of loss in deviation. To this end, it is feasible to compare the deviation ofv the carrier component in the output from a heterodyne frequency modulator with the deviation of the frequency-modulated input signal which produces it'. From this comparison it is possible to derive a control signal proportional to the difference in deviation between the output carrier and the modulated input signal. This signal, in turn, may be used to modify the operation ofthe heterodyne frequency-modulated oscillator in a manner to compensate for the failure of the system to maintain the frequency of the locked sideband component essentially fixed. For example, the control signal maybe applied to vary the normal frequency of oscillation of the oscillator. In the case of a magnetron, for example, this might be effected by using the control signal to vary the anode voltage of the magnetron, or by applying the control signal to Athe frequency control element of an electricallydeviated magnetron of the sort described in Proceedings of the Institute of Radio Engineers AforJuly, 1947, page 657. Alternatively, compensation of the output signal from the heterodyne frequency-modulated oscillator might be achieved by varying the tuning of the auxiliary tank cir- .cuit in an arrangement such as the one shown in Figure 3. For this purpose it would likewise be feasible to utilize the electronic method of varying the tuning of a cavity resonator described in the` aforementioned issue of Proceedings of the Institute of Radio Engineers, page 644 et seq.

` A block diagram of a typical arrangement for controlling the frequency of oscillation of a heterodyne frequency-modulated oscillator, in accordance with the method above outlined, so as to minimize the difference in frequency deviation between the modulated input signal to the oscillator and the modulated output therefrom, is illustrated in Figure 6. In'this arrangement a portion of the output from heterodyne rfrequency- ,modulated oscillator 60 is supplied to mixer 6|.

The mixer is also supplied with a suitable signal from local oscillator 62 andV is adaptedr to `convertthe portion ofthe modulated output from heterodyne frequency-modulated,oscillator 60 to 14 a lower carrier frequency, which is preferably substantially the same as the carrier frequency of the modulated input signal to the heterodyne frequency-modulated oscillator. The heterodyne output from mixer 6| is supplied to a discriminator circuit 63, which is tuned to the carrier frequency of the output signal from the mixer, and which is adapted to produce an output which varies in accordance with the deviation of the signal thus supplied to it. Another similarly tuned discriminator 64 is supplied with a portion of the frequency-modulated I.-F. input signal supplied to heterodyne frequency-modulated oscillator 60 and is adapted to produce an output whose magnitude varies in accordancewith the deviation of the frequency-modulated I.-F. input signal. The outputs from discriminators 63 and 64 are compared in a conventional comparator 65, which is adapted to develop a control signal proportional to any difference in their magnitudes. This control signal is supplied through connection 66 to control heterodyne frequency-modulated oscillator 69, by any of the methods above referred to, so as to compensate for the difference in deviation between the lfnput signal thereto and the output signal thererom.

It will be noted that when the deviation of the modulated output signal from heterodyne frequency-modulated oscillator Gil is the same as the deviation of the modulated I.-F. input signal supplied thereto, the outputs from discriminators 63 and B4 will be the same and no control signal will be produced by comparator $5. However, if there is a difference in deviation between the input signals to heterodyne frequency-modulated oscillator 60, and the output signals therefrom, the outputs from discriminators 63 and @il will differ and a control signal will be developed by comparator 65 which will tend to correct this condition. It will also be noted that the provision of mixer 6l, for converting the modulated output from heterodyne frequency-modulated oscillator 6|] to a lower carrier frequency for supply to discriminator 63, permits the use of discriminators 63 and 64 tuned to the same center frequency. Thus, there is avoided the possibility of unbalance in the system which might result from the use of discriminators tuned to different center frequencies.

It will of course be understood that the arrangement just described with reference to Fig. 6 represents but one of the possible ones for modifying the operation of the heterodyne frequencymodulator in response to the output therefrom so as to counteract for the failure of the modulator to maintain the frequency of the locked sideband component substantially fixed. Other equally effective arrangements for achieving this result will occur to those skilled in the art for achieving these results in accordance with the general principles as above set forth. These improvements, including the embodiment just described, are the subject of a separate copending application Serial No. 61,979, filed November 26, 1949, by Stephen W. Moulton for an Electrical System, which contains claims directed to them.

In Figure 7 is illustrated, schematically, the circuit of a heterodyne frequency-modulated oscillator in which locking of the selected sideband is achieved by injecting, into the oscillator tank circuit, a continuous Wave signal of frequency substantially equal to the mean frequency of the sideband to be locked. In this circuit arrangement a magnetron 10 is driven, in the same manner as I hereinbefore described with reference to Figure 3, by Aa frequency-modulated I.F. input signal supplied to the cathode of the magnetron through a driving circuit including tube 'l l, and which otherwise may be identical to that illustrated in Figure 3 for driving magnetron E i. A locking signal, of frequency equal to the mean frequency of the sideband to be locked in the output from magnetron 1D, is supplied by a suitable c-iv oscillator i2, which may be of conventional construction, through a buffer amplifier i3 and 'transmission line section 74, and is injected into a cavity of the magnetron through the agency or" coupling loop 15. Oscillator 'i2 should be capable of supplying sufficient power to insure locking of the desired sideband, the amount of power required under .any given set of circumstances being readily determinable by experiment. Buffer amplifier F3 should be designed to provide adequate isolation to prevent oscillations: in the magnetron from "pulling the frequency of oscillator i2. The criteria for satisfactory locking hereinbeforc mentioned are equally applicable Ythis instance, excepting, of course, the ones relating to the characteristics of the auxiliary tuned circuit ich is not used in this embodiment. in the operation of the present circuit, when satisfactory locking is eected, the fundamental or carrier frequency in ,the output of magnetron "iii will be caused to vary in frequency in substantially the same manner as the intermediate frequency input signal. Useful power output represented by this component is conveniently derivable from a cavity of the inavnetron by means of a suitable coupling; loop iS and transmission line section It has already been observed that, under certain circumstances, in a heterodyne frequency modulation system according to the invention, there may be a tendency for a forn of distortion to appear in the modulated output signal. As was mentioned, this is particularly lilel to be the case where the amount of power in the fundamental or carrier frequency component is made large, compared to that in the locked sideband component, in order to secure greater efficiency of operation. A method of and means avoiding or minimizingl this distortion, while maintaining high efiicienoy, has already befr; discussed. Under certain circumstances i'. refinement in the system, directed toward the elimination or minimization of another form of distortion, may be desirable. This further refinement will now be discussed.

In a heterodyne frequency-modulation system, if the variations in the frequency-modulated input signal occur relatively slowly, and if the locked sideband is maintained substantially xed in frequency, the variations in frequency of the output signal will be almost directly proportional to the variations in frequency of the modulated input signal. However, as the rate of variation of the frequency of the modulated input signal increases, this proportionality will tend to be destroyed. More particularly, it is found that the variations in the output signal will no longer be proportional solely to the variations in the input signal, but will be dependent also upon the rate at which the input signal varies. In other words, the variations in the output signal will tend to become a function of the rate of change with time of the modulated input signal. This constitutes a form of distortion in the output signal, the nature of which can best be appreciated by considering the frequency-modulated input signal as consisting of a fixed frequency component (i. e. the nominal I.F. input frequency) plus a variable component representative of the modulation. Similarly the frequency-modulated output signal may be regarded as consisting of a fixed component (i. e. the carrier frequency) plus a variable component representative of the modulation. It is then found that there exists an expression relating the variable component of the frequency-modulated output signal to the variable component of the frequency-modulated input signal, the instantaneous frequency of the input signal and the rate of change thereof with time. In the case where the first lower sideband is locked, this expression is:

t*" warum-m2' d: (3) in which pour is the variable component of the frequency-modulated output signal um is the variable component of the frequency-modulated input signal, 'yu is the half bandwidth of the loaded primary in the equivalent circuit of Figure 1, u is the magnitude of the displacement of the locked sideband from the resonant frequency of the primary, and .om is the instantaneous frequency of the input frequency-modulated signal which varies with time. The latter term in this expression represents the distortion component in the frequency-modulated output signal which, as above mentioned,v is a function both of the instantaneous value of the frequency-modulated input signal and the iirst derivative thereof with respect to time. It will be apparent that the distortion term increase with an increase in the rate of variation of the frequency-modulated input signal as well as with the reduction in the difference between wm and c.

It has been determined that, at least under certain circumstances, this form of distortion can be eliminated or substantially reduced through the expedient of predistorting the frefluency-modulated input signal before it is applied to modulate the oscillator. Thus, it has been determined that, in a heterodyne modulated oscillator operated Class C at a relatively low duty cycle, and where the rst lower sideband in the output from the oscillator is locked, the necessary predistortion can be eiected, to achieve complete compensation, by passing the modulated input signal through a frequency discriminatory passive transducer having a transfer irnpedance characteristic given by the expression:

where 1c is an arbitrary real constant, and yn, om and p. are as hereinbefore defined with reference to Equation 3.

Similarly, in the case in which the first upper sideband is locked, the desired compensation may be accomplished completely by passing the frequency-modulated input signal through a passive transducer whose transfer impedance characteristic is given by the expression:

Where the symbols used are of the saine significanoe as in the previous expression.

In practice, the predistorting transducer may comprise a portion of the amplifier through which the intermediate frequency signal is supplied to drive the oscillator which is heterodynenlodulated. To this end, the coupling network between successive amplifier stages may be designed in a manner to yield the desired distortion. For example, where the first lower sideband is locked, it has been determined that, if the coupling network is made to consist of a simple series tuned circuit, with dissipation, as in the arrangement according to Figure 8, a high degree of compensation will result even though this relatively simple form of coupling network provides but an approximation to the transfer impedance characteristic as prescribed by Equation 4. The attainment of'such results is, ofcourse, predicated upon the appropriate selection of the values of the coupling network parameters, but, as will presently become apparent, these values are not very critical.

In the coupling network according to Figure 8, the impedance ZAB between points A and B is given, in accordance with well-known principles of network theory, by the expression:

ZAR:Kiwi@-worwjiwwm (6) The form of this expression is such, it will be observed, that the impedance defined by it is characterized in having two complex conjugate zeroes and a real pole.

If it is assumed that Lit-:M the magnitude cf the displacement of the locked lower sideband from the resonant frequency of the primary, in Equation 4; and that y=y11, the half-bandwidth of the loaded primary, then, if the bandwidth of the intermediate frequency is not unduly wide, distortion in the output of the heterodyne modulated voscillator' will become substantially nonexistent if the value of the parameter a in Equation 6 is selected so as to satisfy the relation:

where wm is the mean intermediate frequency.

If, for example, wn 1=uv=y1n the value of a. will be approximately 0.209. This is within the range of values of a which are susceptible of physical realization, since a= when Rz=O and a:2 when R1=0. j

Once the values of the parameters y, w.- and a have been ascertained for any specic case, it is readily possible, fromv Equations 7 to determine thevalues of L, C, R1', and R2 in the coupling network according to Figure 8. Before doing this, however, it is first necessary to specify the impedance level of the coupling network between points A and B. For practical purposes, this level isvlimited by the inherent capacitance with respeci; to ground of amplifier tube 80 and the gridto-ground capacitance of tube 8|.

For example, if tubes 80 and 8l are type GAKG and are operated at an intermediate frequency of 50 megacycles, values of the circuit constants which will satisfy Equations 6 and provide a suitable impedance level are:

To provide adequate gain, coupling condenser Cc should be of the order of magnitude of 50 up. fd.

The method and means just discussed, for predistorting the input signal to a heterodyne frequency-modulatcr in a manner to reduce the tendency of the output signal therefrom to contain frequency variations which are a function of the rate of change with time of the frequency of the input signal, are claimed in copending application of Stephen W. Moulton, Serial No. 65,301, filed December 12, 1948, now U. S. Patent No. 2,529,736 issued November 14, 1950, for an Electrical System.

I claim:

1. In a modulating system, an oscillator having a tank circuit, means responsive to a signal of modulated frequency for controlling said oscillator in a manner to produce in said tank circuit a plurality of frequency components, at least one of which tends to be frequency-modulated in some degree in substantial accordance with the modulation of said signal and at least another of which is of substantially greater magnitude than said one component, a resonant circuit coupled to said tank circuit, said resonant circuit being tuned to substantially the mean frequency of said one component and being operative to reduce the tendency of said one component to be frequencymodulated and to impart to said other component frequency variations according in some degree to the modulation of said one component which would have existed in the absence of said reduction, and means for deriving from said oscillator'` an output signal which varies in frequency in accordance with the frequency variations of said other component.

2. In a modulating system, an oscillator having a tank circuit, means 'responsive to a signal of modulated frequency for controlling said oscil` lator in a manner to produce in said tank circuit fundamental and sideband frequency components, said fundamental component being of appreciably greater magnitude than said sideband components and at least one of said sideband components having a tendency to be frequency-modulated inl some degree in substantial accordance with the modulation of said signal, a resonant circuit coupled to said tank circuit, said resonant circuit being tuned to substantially the mean frequency of said one sideband component and being operative to reduce the tendency of said one component to be frequency-modulated and to impart to said fundamental component frequency variations according in some degree to the modulation of said one component which would have existed in the absence of said reduction, and means for deriving from said tank circuit an output signal which varies in frequency in accordance with frequency variations of said fundamental component.

3. In a modulating system, an oscillator having a tank circuit, means responsive to a signal of modulated frequency for controlling said oscillator in a manner to produce in said tank circuit a plurality of signal frequency components, at least one of which tends to be frequencymodulated in some degree in substantial accordance with the modulation of said signal, an

auxiliary source of energy of a frequency closely approximating the mean freniency` of said one component, means for supplying energy from said source to said tank circuit to reduce the tendencyof said one .component-to be frequency-modulated and to impart to another of said comiii ponents frequency variations according in some degree to the modulation of said one component which would have existed in the absence of said reduction, and means for deriving from said tank circuit an output signal which varies in frequency in accordance with frequency variations of said other component.

4. In a modulating system, an oscillator, a signal utilization device, a waveguide coupling said oscillator to said utilization device and adapted to supply signals generated by said oscillator to said Yutilization device, means Yresponsive to a signal of modulated frequency for controlling said oscillator 'in a manner -to produce therein a plurality of signal frequency cornponents, at least one of Whichtends to be frefluency-modulated in some degree in substantial accordance with the modulation of said firstnamed signal, and a cavity resonator coupled to said waveguide at a point interjacent said oscillator and said utilization device for reducing the tendency of said one component to be frequencymodulated and for imparting to another of said components frequency variations according in some degree to the modulation of said one component which would have existed in the absence i of said reduction, said resonator being tuned to substantially the vmean frequency of said one component and having a Q substantially greater than the Q of said tank circuit, Q denoting the ratios of reactance to resistance of said circuits respectively.

5. In a modulating system, a cavity magnetron oscillator, means responsive to a signal of modulated frequency for modulating said oscillator in a manner to produce therein a plurality of signal frequency components, at least .one of which tends to be frequency-modulated in some degree in substantial accordance with the modulation of said signal, an auxiliary oscillator for producing' a signal whose frequency approximates the mean frequency of said one component, means including a buffer amplifier for supplying signal from said auxiliary oscillator to said magnetron oscillator, whereby to reduce the tendency of said one component Vto be frequency-modulated and to impart to another of said components frequency variations according Vin some degree to the modulation of said one component which would have existed in the absence of said reduction, and means for deriving from said magnetron an output signal which varies in frequency in accordance with frequency variations in said other component.

6. In a modulating system, an oscillator having a tank circuit, means .responsive to a signal of modulated frequency for controlling said oscillator in a manner to produce in said tank circuit a plurality of signal frequency components, at least one of which tends to be frequency-modulated in some degree in substantial accordance with the modulation of said signal, a resonant circuit coupled to said tank circuit, said resonant circuit having a Q substantially greater than the Q of said tank circuit, Q denoting the ratios of reactance to resistance of said circuits respectively, and being tuned to substantially the mean frequency of said one component and being adapted to reduce the tendency of said one component to be frequency-modulated and to impart to another of said components frequency variations according in some degree to the modulation of said one component which would have existed in the absence of said reduction, and means for deriving from said tank circuit an output signal which varies in frequency inaccordance with frequency variations of said other component.

7. In a modulating system, an oscillator' having a, tank circuit, means responsive to a signal of modulated frequency for controlling said oscillator in a manner to produce in said circuit a plurality of signal frequency components, at least one of which tends to be frequency-modulated insome degree in substanital accordance with the modulation of said signal, resonant circuit tuned to substantially the mean frequency of said one component, means coupling said resonant circuit to said tank circuit in a manner to reduce the tendency of said one component to be frequency-modulated and to impart to another of said components frequency variations according in some degree to the modulation of said one component which would have existed in the sence of said reduction, said means having an admittance Whose phase angle o is substantially that given by the expression:

where w11 is the resonant frequency of the loaded oscillator tank circuit, m is the resonant frequency or said resonant circuit when loaded, 7n is the half bandwidth of the loaded oscillator tank circuit and w22 is the half bandwidth of said resonant circuit when loaded, and means for deriving from said tank circuit an output signal which varies in frequency in accordance with frequency variations of said other component.

8. In a modulating system, an oscillator having a tank circuit, means responsive to a signal of modulated frequency for controlling said oscillator in a manner to produce in said tank circuit a plurality of signal frequency components, at least one of which tends to be frequency-modulated in some degree in substantial accordance with the modulation of said signal, a tuned circuit resonant at substantially the mea-n frequency of said one component, means magnetically coupling said tuned circuit to said tan-` circuit whereby said tuned circuit `is effective to reduce the tendency of said one component to be frequency-modulated and to impart to another of said components frequency variations according in some degree to the modulation of said one component which Would have existed in the absence of said reduction, the magnitude of the magnetic coupling provided by said means being such as to maintain the relation:

ci: n Q22 Q11 Where Q10 is the Q of the unloaded oscillator tank circuit, Q11 is the Q of the loaded oscillator tank circuit, Q20 is the Q of said last named resonant circuit when not loaded and Q22 is theQ -o'fsaid last-named resonant circuit when loaded, Q denoting the ratios of reactance to resistance of said'circuits respectively, and means for deriving from said tank circuit an output signal .which varies in frequency in accordance with frequency variations of said other component.

9. In a modulating system, a source of a signal of predetermined frequency, a source of a second signal of modulated frequency, means including a tank circuit for mixing said first and second signals to yield a third signal comprising a plurality of frequency components, at least `one of which tends to be frequency-modulated in some degree in substantial accordancevwith the frequency modulation of said second. signaland resonant circuit coupled to said tank circuit, 'said resonant circuit being tuned to substantially the mean frequency of said one component and being operative to reduce the tendency of said one component to be frequency-modulated and to impart to said other component frequency variations according in some degree to the modulation of said one component Which would have existed in the absence of said reduction, and means for deriving from said oscillator an output signal which varies in frequencyin accordance with the frequency variations in said other component.

10. In a modulating system, a source of a signal of predetermined frequency, a source of a second signal of modulated frequency, means including a tank circuit for mixing said rst and second signals to yield a third signal comprising a plurality of frequency components, at least one of Whichtends to be frequency-modulated in some degree in substantial accordance with the frequency modulation of said second signal and at least another of Which is of substantially greater magnitude than said one component, an.

auxiliary source of energy of a frequency closely approximating the mean frequency of said one component, means for supplying energy from said source to said tank circuit to reduce the tendency of said one component to be frequencymodulated, whereby there are imparted to said other component frequency variations according in some degree to the modulation of said second signal which Would have existed in the absence of said reduction, and means for deriving an output signal which varies in frequency in accordance with the frequency variations of said other component.

11. In a modulating system, a source of a signal of modulated frequency, an oscillator having a resonant tank circuit, the half-bandwidth of said tank circuit when loaded being of the same order as the mean frequency of said frequencymodulated signal, means responsive to said signal for controlling said oscillator in a manner to produce, in the output from said oscillator, a plurality of frequency components, at least one of which tends to be frequency-modulated in some degree in substantial accordance with the modulation of said signal, a resonant circuit coupled to said tank circuit, said resonant circuit being tuned to substantially the mean frequency of said one component for substantially reducing the tendency of said one component to be frequency modulated, whereby there are imparted to another of said components frequency variations according in some degree to the modulation of said one component which would have existed in the absence of said reduction, and means for deriving an output signal which varies in frequency in accordance With the frequency variations of said other component.

12. In a modulating system, a source of a signal of modulated frequency, an oscillator having a resonant tank circuit, the half-bandwidth of said tank circuit when loaded being of the same order as the mean frequency of said frequencymodulated signal, means responsive to said signal for controlling said oscillator in a manner to produce, in the output from said oscillator, a plurality of frequency components, at least one of which tends to be frequency-modulated in some degree in substantial accordance with the modulation of said signal, an auxiliary source of energy of a frequency closely approximating the mean frequency of said one component, means for supplying energy from said source to said tank circuit to reduce the tendency of said one component to be frequency-modulated, whereby there are imparted to another of said components" frequency variations according in some degree to the modulation of said one component which Would have existed in the absence of said reduction, and means for deriving an output signal which varies in frequency in accordance with the frequency variations of said other component.

13. In a modulating system, a source of a signal of predetermined frequency, a source of a second signal of modulated frequency, means including a tank circuit for mixing said first and second signals to yield a third signal comprising fundamental and sideband frequency components, said fundamental component being of appreciably greater magnitude than said side'- band components and at least one of said sideband components having a tendency to be frequency-modulated in some degree in substantial accordance With the frequency modulation of said second signal, a resonant circuit coupled to said tank-circuit, said resonant circuit being tuned to substantially the mean frequency of said sideband component for substantially reducing the tendency of said sideband component, to be frequency-modulated, whereby there are imparted to said fundamental component frequency variations according in some degree to the modulation of said sideband component which would have existed in the absence of said reduction, and means for deriving an output signal which varies in frequency in accordance with the frequency variations of said fundamental component.

14. In a modulating system, a source of a signal of predetermined frequency, a source of a second signal of modulated frequency, means including a tank circuit for mixing said first and second signals to yield a third signal comprising fundamental and sideband frequency components, said fundamental component being of appreciably greater magnitude than said sideband components and at least one of said sideband components having a tendency to be frequency-modulated in some degree in substantial accordance with the frequency modulation of said second signal, an auxiliary source of energy of a frequency closely approximating the mean frequency of said sideband component, means for supplying energy' from said source to said tank circuit to reduce thetendency of said sideband component to be fre-l quency-modulated, whereby there are imparted to= said fundamental component frequency varia-f tions according in some degree to the modulation of said sideband component which would have existed in the absence of said reduction, and means for deriving an output signal which varies in frequency in accordance With the frequency variations of said fundamental component.

15. In a modulating system, an oscillator having a tank circuit, means responsive to a signal of modulated frequency for controlling said oscillator in a manner to produce in said tank circuit a plurality of frequency components, at least one of which tends to be frequency-modulated in some degree in substantial accordance With the modulation of said signal and at least another of Which is of substantially greater magnitude than said one component, an auxiliary source of energy of a frequency closely approximating the mean frequency of said one component, means for supplying energy from said source to said tank circuit to reduce the tendency of said one compllellt to be frequency-modulated, whereby there are imparted to said other component frequency7 variations according in some degree to the mcdulation of said one component which would have existed in the absence of said reduction, and means for deriving from said oscillator an output signal which varies in frequency in accordance Withthe frequency variations of said other cornponent.

16. In a modulating system, an oscillator having a tank circuit, means responsive to a signal of modulated frequency for controlling said oscillator in a manner to produce in said tank circuit fundamental and sideband frequency components, said fundamental component being of appreciably greater magnitude than said sideband components and at least one of said sideband components having a tendency to be frequency-modulated in some degree in substantial accordance With the modulation of said signal, an auxiliary source of energy of a frequency closely approximating the mean frequency of said last-named sideband com- 24 ponent, means for supplying energy from said source to said tank circuit to reduce the tendency of said last-named sdeband component to be frequency-modulated and to impart to said fundamental component frequency variations according in some degree to the modulation of said sideband component which would have existed in the absence of said reduction, and means for deriving from said tank circuit an output signal which varies in frequency in accordance with frequency variations of said fundamental component.

WILLIAM E. BRADLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,027,975 Hansell Jan. 14, 1936 2,052,576 Lindenblad Sept. 1, 1936 2,135,199- Ponte et al Nov. 1, 1938 2,421,725 Stewart June 3, 1947 

